1. Field of the Invention
The present invention relates to storing permanent narrow linewidth volume holograms in photorefractive materials and more specifically to narrow bandwidth (sub-Angstrom) filters using volume holograms and methods of making the same.
2. Description of Related Art
Methods of writing and fixing high diffraction efficiency, counter-propagating reflection holograms in photorefractive materials have been described by the above-referenced parent patent applications. One example of use of this technology is extremely narrow bandwidth reflection filters with sub-Angstrom full-width-at-half-maximum (FWHM) response. Such filters can be used, for example, in solar astronomy to image the sun at H.sub..alpha. or other absorption bands. Improved filters are fabricated by recording curved (i.e., spherical) volume holographic gratings in LiNbO.sub.3 and fixing the holograms to render them permanent so they will not be erased by illumination during use or by thermal decay during storage or shipping. The parent patent applications by G. A. Rakuljic and A. Yariv, supra, also describe an improved method of fixing holograms written in photorefractive materials such as LiNbO.sub.3 with very high fixing efficiency, i.e., with little loss in diffraction efficiency in the conversion from the pre-fixed to permanent gratings.
Holograms were first fixed in LiNbO.sub.3 by a group at RCA Laboratories (J. J. Amodei and D. L. Staebler, "Holographic recording in lithium niobate," RCA Review, vol. 33, pp. 71-94 (1972) ). The fixing methods of the parent patent applications by G. A. Rakuljic and A. Yariv, supra, which have been verified by a systematic analysis of the photorefractive charge transport equations developed by N. V. Kukhtarev, et al. ("Holographic storage in electro-optic crystals. I. Steady state," Ferroelectrics vol. 22, pp. 949-960 (1979)), involve control of temperature and electric fields in the crystal to obtain high fixed diffraction efficiencies. Holograms are stored in photorefractive materials in the form of an electronic grating resulting from light-induced migration of photo-excited electrons. The electro-optic properties of the material induce index of refraction variations generated by the spatial electric field distribution resulting from the electronic grating. The fixing and developing processes involve generating an ionic grating to compensate the metastable electronic grating, then removing the original electronic grating to leave only a permanent ionic grating that cannot be erased by illumination. This is done by first heating the crystal to a temperature where the ions become mobile so they migrate to compensate the electronic grating, and controlling the electric fields in the crystal during the process to achieve the highest possible ionic grating strength. The main steps in this process, as described in the parent patent applications by G. A. Rakuljic and A. Yariv, supra, are:
The grating is written in the crystal while it is shorted to neutralize all internal fields. A transparent conducting material such as carbon film or thin metal layer is used to coat the surfaces of the crystal in order to short it while allowing light through its faces. This generates an index grating generated by an electronic space charge distribution that will decay with time or illumination. PA0 The crystal is heated to its fixing temperature, usually around 160.degree. C., while it is shorted by being wrapped in foil so the ions become mobile and compensate the electron grating. The crystal is held at the high temperature for a few minutes, and cooled back to room temperature. PA0 The foil is removed and the crystal cleaned so it becomes a good insulator. The crystal is illuminated by intense light while held in the open circuit state to allow large fields to build up in the crystal through the photovoltaic effect and to enhance the erasure of the original electronic grating, leaving only the permanent ionic grating (developing). PA0 The crystal is shorted with a transparent conducting material as in the previous example of sequential writing and fixing and heated to the fixing temperature of approximately 160.degree. C. PA0 The grating is written in the crystal at the elevated temperature and cooled back to room temperature. PA0 The shorting material is removed and the crystal illuminated by an intense light while open circuited as in the previous example.
The preceding steps describe the sequential writing and fixing process. An alternate process, the simultaneous writing and fixing case, comprises the following steps:
These novel processes allow very narrow bandwidth holograms to be established in thick (up to 8 mm thick) crystals with high dopant concentrations (0.1% Fe). As an unexpected consequence, however, peak diffraction efficiencies and bandwidth characteristics are significantly below theoretical expectations. In some instances, wavelength shifts, multiple peaks, or bandwidth broadening beyond the width of the pre-fixed grating have been observed after the developing step. In other instances, these effects have been encountered after a crystal is used in the solar filter application for a period of time. One wishes to retain the high dopant level in a thick crystal so as to achieve a high diffraction efficiency, but it is also of significant importance that the extremely narrow band filter properties be retained during and after development.